Abnormal blood vessel formation contributes to the pathogenesis of numerous diseases with high morbidity and mortality. Elucidation of the mechanisms underlying vascular growth might allow the development of therapeutic strategies to stimulate vascular growth in ischemic tissues or to suppress their formation in tumors. Recent gene targeting studies in embryos have identified some of the mechanisms involved in the initial formation of endothelial channels (angiogenesis) and their subsequent maturation by coverage with smooth muscle cells (arteriogenesis). Evidence is emerging that distinct molecular mechanisms may mediate growth of blood vessels during pathological conditions, but the molecular players remain largely undetermined.
VEGF has been implicated in development and pathological growth of the vasculature (N. Ferrara et al., 1999, Curr. Top. Microbiol. Immunol. 237, 1-30). Deficiency of a single VEGF allele causes fatal vascular defects (P. Carmeliet et al., 1996, Nature 380, 435-439; and N. Ferrara et al., 1996, Nature 380, 439-442), whereas suppression of VEGF in the neonate or expression of a single VEGF 120 isoform results in impaired vascular growth (H. P. Gerber et al., 1999, Development 126, 1149-1159; and P. Carmeliet et al., 1999, Nat. Med. 5, 495-502). In the adult, VEGF affects vascular growth during reproduction, wound healing, and malignant and inflammatory disorders (N. Ferrara et al., 1999, Curr. Top. Microbiol. Immunol. 237, 1-30). VEGF is currently being tested for therapeutic angiogenesis in the ischemic heart and limb, but initial clinical trials have resulted in both promising and disappointing results (J. M. Isner et al., 1999, J. Clin. Invest. 103, 1231-1236). An outstanding question is whether VEGF is able to stimulate the maturation of vessels with a smooth muscle coat (arteriogenesis). Naked endothelial channels remain vulnerable to traumatic insults, regress during changes in oxygen, and lack vasomotor control to accommodate changes in tissue perfusion (L. E. Benjamin et al., 1998, Development 125, 1591-1598). In some diseases such as pulmonary hypertension, excess arteriogenesis is an undesired and poorly controllable phenomenon. In pulmonary hypertension, remodeling of the pulmonary vasculature occurs because vascular smooth muscle cells proliferate and migrate distally around the terminal arterioles, increasing thereby the pulmonary vascular resistance. Another aspect of VEGF is that this molecule affects the permeability and growth of adult quiescent vessels. In normal human serum, no detectable levels of VEGF are present, but under pathological conditions, such as cancer and inflammatory disorders, VEGF is highly up-regulated and mediates the formation of undesired edema. Edema formation is also an important clinical problem associated with several tumors leading to ascites in peritoneal tumors, pleuritis in lung cancer and cerebral edema in brain tumors (possibly leading to fatal intracranial hypertension) and often facilitates metastasis of tumors. Vascular congestion and edema are important pathogenic mechanisms in asthma, brain infarct expansion after stroke, peritoneal sclerosis after dialysis or abdominal interventions, etc. Other VEGF homologues have been identified, but their role in angiogenesis and arteriogenesis remains unclear.
One interesting homologue of VEGF is Placental Growth Factor (PIGF) but its role in vascular growth and pathogenesis has been poorly studied (M. G. Persico et al., 1999, Curr. Top. Microbiol. Immunol. 237, 31-40). U.S. Pat. No. 5,919,899 describes PIGF and its use in the treatment of inflammatory disorders, wounds and ulcers. Donnini et al. (J. Pathol. 189, 66, 1999) have observed a correlation between up-regulation of PIGF and human meningiomas but it is clear that there is no indication whatsoever that PIGF has a role in tumor formation. The role of PIGF in edema was studied by Monsky et al. (Cancer Res. 59, 4129, 1999), but no in vivo role for PIGF in edema formation during pathological processes could be found in several mouse and human tumors.
Inhibitors for PIGF are not known in the art except for a goat polyclonal antibody against human PIGF (R&D Pharmaceuticals, Abingdon, UK) and a chicken polyclonal antibody (Gassmann et al., 1990, Faseb J. 4, 2528). Those antibodies are used for western blotting, histochemistry and immunoprecipitation studies. The role of the PIGF receptor (=VEGFR-1) for endothelial cell biology has also remained enigmatic (A. Sawano et al., 1996, Cell Growth Differ. 7, 213-221 and M. Clauss et al., 1996, J. Biol. Chem. 271, 17629-17634). Gene-targeting studies yielded conflicting results on the role of VEGFR-1, either as a possible signaling receptor (suggested by the vascular defects in VEGFR-1-deficient embryos (G. H. Fong et al., 1999, Development 126, 3015-3025)) or as an inert binding site, a “sink,” for VEGF, regulating availability of VEGF for the angiogenic VEGFR-2 (suggested by the normal vascular development in mice expressing a truncated VEGFR-1, lacking the tyrosine kinase domain (S. Hiratsuka et al., 1998, Proc. Natl. Acad. Sci. U.S.A. 95, 9349-9354)).
The present invention relates to the surprising finding that PIGF is a specific modulator of VEGF during a variety of pathological conditions, such as ischemic retinopathy, tumorigenesis, inflammatory disorders, wound healing, edema and pulmonary hypertension. This finding has implications for the inhibition of vascular leakage (edema formation), inflammatory disorders, tumor formation, pathological angiogenesis and the prevention of pulmonary hypertension that occurs during pathological arteriogenesis.